Year	Years
1970	2
1971	2.04
1972	2.09
1973	2.14
1974	2.2
1975	2.25
1976	2.3
1977	2.35
1978	2.41
1979	2.46
1980	2.52
1981	2.58
1982	2.63
1983	2.69
1984	2.75
1985	2.81
1986	2.87
1987	2.92
1988	2.98
1989	3.04
1990	3.1
1991	3.16
1992	3.22
1993	3.28
1994	3.33
1995	3.39
1996	3.44
1997	3.49
1998	3.55
1999	3.6
2000	3.66
2001	3.73
2002	3.79
2003	3.86
2004	3.93
2005	4
2006	4.07
2007	4.15
2008	4.22
2009	4.3
2010	4.38
2011	4.47
2012	4.55
2013	4.64
2014	4.73
2015	4.82